Electrochemical energy storage devices that offer a combined high energy and power are necessary to meet the demand for future mobile applications. Electrode materials that lead to pseudocapacitive charge storage are of particular interest as they enable Faradaic charge transfer at high rates. This behavior is observed when redox processes are not kinetically limited by the solid-state diffusion of reacting ionic species. Pseudocapacitance has mostly been introduced to transition metal oxides by nanostructuring, thereby increasing the electrode-electrolyte interface and reducing diffusion distances.¹ Besides often costly synthesis procedures, the materials can show a low density and are hard to integrate in macroscopic electrode structures. A new approach to transition from battery-like to pseudocapacitive charge storage behavior is the use of layered metal oxides with interlayer molecules like structural water.²

Hydrogen titanates (HTO) can be obtained by a proton exchange reaction of sodium titanate in acidic solution. Monoclinic H₂Ti₃O₇ consists of layers of edge-sharing TiO₆ octahedra with hydrogen positioned in the interlayer. Upon heating, the interlayer deprotonates over three discrete steps with the intermediate phases H₂Ti₆O₁₃ and H₂Ti₁₂O₂₅, before forming TiO₂(B) (Fig. 1).³ The intermediate phases exhibit reduced numbers of hydrogen atoms in the interlayer. This designates HTO as an ideal model system to study the influence of interlayer structural protonation on the electrochemical intercalation properties.

In this study, we synthesized HTO in different states of reduced interlayer structural protonation by thermal annealing and confirmed the compositions by matching the associated mass loss to the release of 0.5 and 0.75 H₂O per H₂Ti₃O₇, respectively. The materials were employed as hosts for proton intercalation in acidic electrolyte. Our results suggest a strong correlation between protonation state of the HTO interlayer and proton storage capacity, with fully protonated H₂Ti₃O₇ offering the highest proton intercalation capacity. We will discuss the influence of interlayer proton mobility on the charge storage properties and correlate the crystal structure changes of the materials with the intercalation process.

1. Augustyn, V., Simon, P. & Dunn, B. Pseudocapacitive Oxide Materials for High-Rate Electrochemical Energy Storage. Energy Environ. Sci. 7, 1597–1614 (2014).
2. Mitchell, J. B., Lo, W. C., Genc, A., LeBeau, J. & Augustyn, V. Transition from battery to pseudocapacitor behavior via structural water in tungsten oxide. Chem. Mater. 29, 3928–3937 (2017).
3. Morgado, E. et al. Multistep structural transition of hydrogen trititanate nanotubes into TiO₂-B nanotubes: A comparison study between nanostructured and bulk materials. Nanotechnology 18, 81–93 (2007).